Random access digital memories are employed in a variety of systems for storing input data information and permitting the reading out of that information at random times. The address location of the data read into the memory and read from the memory generally is determined by digitally or binary encoded address lines which commonly are in the form of the outputs of a standard binary counter.
Every random access memory has some finite capacity, and, in some applications, it is possible to exceed that capacity and lose data which it is desired to have stored, or the memory address counter may be recycled to cause the storage of new data in address locations before the previously stored data at those locations has been read out of the memory or utilized. In such systems, it is common to employ a digital display, or the like, to tell the operator of the system when the memory is full or near full. Utilization of such a display, however, requires the operator to know the capacity of the memory. Then, he must relate this capacity of the memory to the display in order to determine how much more information can be supplied to the memory or to determine the address location being used relative to the maximum capacity of the memory. This is inconvenient at best and is a significant disadvantage when the user of the system is not technically sophisticated or is not familiar with the maximum memory capacity of the system.
Other systems employ an alarm in the form of a buzzer or a light which is energized as the memory is filled or is about to reach its maximum capacity. In systems of this type, however, it often is too late for the operator to make suitable adjustments for redirecting inputs to some other memory or for terminating the supply of data to the memory on the relatively short notice which necessarily exists when such an on/off type of alarm is employed. Until the memory reaches its capacity or near capacity, the user of a system employing an on/off type of alarm has no idea where the information he is supplying into the memory or reading from the memory is addressed or located relative to the total capacity of the memory. As a consequence, it is difficult for the operator to use such a system with assurance that he is utilizing it most efficiently. Generally, the tendency is to build the system with memory overcapacity to avoid reaching the alarm or memory full condition. This is expensive and inefficient.
Accordingly, it is desirable to provide a system for random access memories or read only memories which provides a continuous indication of the degree of "fullness" of the memory, or an indication of the position of a given address location relative to the full memory capacity. It further is desirable to provide such an indication of memory fullness which is readily understandable by users who are not technically sophisticated.